Method for preparing acrylonitrile

ABSTRACT

This invention relates to a method for preparing acrylonitrile by ammoxydation using a multi-story quenching tower. The method comprises the steps of supplying the reacted gas into the first quenching chamber of the multi-story quenching tower, said first quenching chamber has a water supply pipe equipped with spray nozzles at a two-dimensional density of more than 2 nozzles/m 2  for the sectional area of the multi-story quenching tower and contacting the reacted gas with 5 T or more of water per 1 T of the reacted gas fed from the nozzles.

This application is a 371 of PCT/JP96/00133 filed Jan. 25, 1996.

TECHNICAL FIELD

The present invention relates to a method for preparing acrylonitrile byammoxydation, more particularly to a method for improving the yield andproduction efficiency of acrylonitrile by sufficiently removing ammoniain the quenching process of a reacted gas.

BACKGROUND ART

The method for preparing acrylonitrile by ammoxydation has been improvedon various points since its success in industrialization and it is saidthat the method has technically matured. Still, there have been madeefforts to improve utility unit consumption and to reduce productioncost by achieving production efficiency.

It is well known that the reacted gas obtained from various rawmaterials contains by-products such as hydrocyanic acid, acetonitrile oraldehyde in addition to acrylonitrile when acrylonitrile is prepared byammoxydation. The by-products react with acrylonitrile in the presenceof unreacted ammonia or react with one another; as a result, highboiling point compounds are produced. The high boiling point compoundscause not only reduction in the yield of acrylonitrile, the targetproduct, but also clogging at various parts in the quenching tower inthe downstream processes. Consequently, the by-products must beimmediately separated from the reacted gas. Especially, the unreactedammonia facilitating undesired reactions must be completely removed assoon as possible.

Conventionally, there has been adopted the method for separatingunreacted ammonia in the form of a salt and simultaneously removingother impurities and the by-products mentioned above by preliminarilycooling the reacted gas produced in a reactor, immediately sending thecooled reacted gas to a quenching tower, and washing and quenching thereacted gas with water containing acids such as sulfuric acid. As one ofthe conventional methods excellent in production efficiency, there hasbeen proposed a method employing a multi-story quenching tower to dividea quenching process into two steps or more, which comprises separatingmost, unreacted ammonia in the form of a salt by contacting the watercontaining a sufficient amount of sulfuric acid to neutralize theunreacted ammonia and simultaneously condensing not all but a part ofthe vapor contained in the reacted gas in the first chamber, andcondensing most of the residual vapor in the second chamber (U.S. Pat.No. 3,649,179).

This method succeeded in reducing the cost of recovering and treatingthe salt because of its higher collectability of ammonia and higherconcentration of the resultant ammonium salt than the conventionalmethods. Yet, the collectability of ammonia achieved by this method isnot sufficient. Accordingly, problems such as an acrylonitrile loss andtrouble caused by clogging due to high boiling point compounds (socalled heavies) have still remained.

DISCLOSURE OF THE INVENTION

The present inventors have made extensive and intensive research tosolve the above problems and found that the collectability of ammonia isremarkably enhanced by adjusting the two-dimensional density of spraynozzles and the amount of supplying water in certain ranges. As aresult, they achieved the present invention.

This invention provides a method for preparing acrylonitrile by reactingpropylene and/or propane, ammonia and molecular oxygen in the presenceof a catalyst and purifying the reacted gas in a multi-story quenchingtower. The method comprises the steps of supplying the reacted gas intothe first quenching chamber of the multi-story quenching tower, saidfirst quenching chamber having a water supply pipe equipped with spraynozzles at a two-dimensional density of 2 nozzles/m² or more of thecross sectional area of the multi-story quenching tower, and contactingthe reacted gas with at least 5 T of water fed from the nozzles per 1 Tof the reacted gas. The unit "T" is a measurement of weight andcorresponds to 1000 kg.

The quenching tower used in the present invention includes a multi-storyquenching tower divided into two or more levels and each level consistsof one chamber as shown in FIG. 1.

The gas obtained by reacting propylene and/or propane, ammonia andmolecular oxygen is first introduced into the first quenching chamber(2) to cool and wash it by the gas with water containing an acid such assulfuric acid. At this time, unreacted ammonia, high boiling pointcompounds, polymers, scattered catalysts and the like are removed. Afterthat, the remaining by-products are removed in the chambers on storiesabove the first.

In the multi-story quenching tower, spray nozzles (17) are arranged onthe end part of a water supply pipe (9). On the upper stories, chambersare filled with porcelain Raschig rings as a packed bed (11).

As the spray nozzles (17), nozzles having the differential pressure offrom 1 to 5 kg/cm² G and the spraying angle of from 90 to 100° can beused, for example, hollow cone type and full cone type nozzles. Thetwo-dimensional density of the spray nozzles (17) in the first chamber(2) must be at least 2 nozzles/m² of the cross sectional area of themulti-story quenching tower. As a result, the areas where each spraynozzle (17) can spray water are preferably overlapped with one anotherso that excellent collectability of ammonia may be obtained. When thetwo-dimensional density is less than 2 nozzles/m², water is not spreadall over the cross sectional area of the quenching tower because thesprayed areas are not preferably overlapped. Therefore, the scatteredwater and the reacted gas are not sufficient contact and the preferablecollectability of ammonia is not achieved. The two-dimensional densityof the nozzles is preferably from 2 to 8 nozzles/m², more preferablyfrom 2 to 5 nozzles/m².

The water supply pipe (9) is arranged above the gas lead in port (19) sothat the spray nozzles equipped on the pipe may face the bottom of thechamber. The distance between the port (19) and the spray nozzles 17 ispreferably 0.5 m or more, more preferably 1.0 m or more although thedistance depends on a diameter of the quenching tower.

Hereinafter, the production method in the present invention will bedescribed by referring to FIG. 1, an example of a multi-story quenchingtower. The quenching tower has a diameter of from 2.5 to 3.0 m and aheight of from 9 to 10 m. In general, the reacted gas obtained byreacting propylene and/or propane, ammonia and molecular oxygen in theproduction of acrylonitrile has a composition as follows:

    ______________________________________    < ingredient >       < vol. % >    ______________________________________    acrylonitrile        6.0-7.0    ammonia              0.0-1.0    propylene and/or propane                         0.2-0.6    acetonitrile         0.1-0.5    hydrocyanic acid     0.8-1.6    non-condensable gas  60.0-67.0    water vapor          25.0-30.0    other materials (acrolein,                         0.0-0.2    high boiling point compounds, etc.)    ______________________________________

This gas is supplied to the first quenching chamber (2) through a gaslead in pipe (1) a rate of from at about 15 to 25 T/Hr at a temperaturefrom 260° to 280° C.

In the first chamber (2), water is supplied to the chamber from thespray nozzles (17) in the direction against the flow of the gas at arate of from 150 to 170 T/Hr. The water contacting the gas may berecycle through a discharging pipe (4) and the water supply pipe (9)into the first quenching chamber again. If necessary, a refrigerator (6)may be used.

The amount of the water supplied to the first chamber (2) must be atleast 5 T per 1 T of the reacted gas. If it is less than 5 T per 1 T ofthe reacted gas, ammonia and high boiling point compounds are notsufficiently collected. As a result, the reacted gas flows up into theabove stories and the high boiling point compounds clog the packed bed(11) and pipes. Further, the water is preferably fed in an amount of 20T or less per 1 T of the reacted gas to obtain desirable collectabilityof ammonia and utility unit consumption. The more preferable amount ofthe water is from 7 T to 10 T per 1 T of the reacted gas.

An acid necessary for neutralization of ammonia, preferably sulfuricacid, is added to the water in the first chamber (2). The acid ispreferably added so as to adjust the pH value of the water preferablyfrom 5 to 6, more preferably from 5.3 to 5.8. By controlling the pHvalue within this range, loss of acrylonitrile can be minimized. Whenthe pH value is too high or low, loss of acrylonitrile increases, andmoreover, the tower, packed beds, pipes and trays are soiled or cloggedin a quenching step and an adsorption step following the quenching step.

Preferably, the reacted gas is supplied to the first chamber (2) at alinear velocity of from 0.10 to 0.90 m/sec. When the linear velocity ofthe reacted gas is greater than 0.90 m/sec., the collectability ofammonia is likely to be reduced because the reacted gas tends to causeentrainment of water into the later steps. When the linear velocity ofthe reacted gas is less than 0.10 m/sec., the effects of the presentinvention are not efficiently attained and the low linear velocity isnot preferable in the aspects of initial investment in plant andequipment. The more preferable linear velocity is from 0.50 to 0.80m/sec.

The reacted gas treated in the first chamber (2) is transferred to thestories above the first (FIG. 1 only shows a two-story quenching tower),and the removal of ammonia in the reacted gas is continued. On the upperstories, the reacted gas can be treated according to the conventionalmethods.

In the case of the quenching tower shown in FIG. 1, the water in thesecond chamber (3) is cooled down to a temperature about 36° to 38° C.with a refrigerator (13) and supplied into the second chamber throughthe water supply pipe (15) at a rate of from 170 to 190 T/Hr. The vaporinitially contained in the reacted gas is only partially condensed inthe first chamer (2), and most of the vapor is liquefied in this secondchamber (3). The water used in the quenching chambers on stories abovethe first can be recycled in each quenching chamber on stories above thefirst. If necessary, water containing an acid can be contacted with thereacted gas in the chambers on the upper stories.

The temperature in the first quenching chamber (2), at the time when theinside of the multi-story quenching tower reaches its equilibrium state,is from 80 to 100° C. although it may change somewhat depending on thetemperature and composition of the reacted gas transferred into thechamber. When the inside of the quenching tower reaches its equilibriumstate, the water in the first quenching chamber (2) becomes a solutioncontaining an extremely small amount of acrylonitrile, ammonium sulfateand viscous high boiling point compounds. The water is discharged fromthe first quenching chamber (2) out of the tower at a rate of from about0.5 to 1.0 T/Hr. The pressure at the bottom of the tower is from about0.4 to 0.6 kg/cm² G.

The reacted gas, from which ammonia is removed in the multi-storyquenching tower, is transferred to an absorption tower. The temperatureof the gas at the transferring pipe (16) is from about 37° to 39° C.

According to the present invention, collectability of ammonia of 90% ormore is achieved and other by-products can be effectively removed whilemaintaining the original effects of the multi-story quenching tower.Consequently, loss of acrylonitrile and clogging of pipes or packed bedsdue to the high boiling point compounds can be prevented.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view of the multi-story quenching tower used inthe present invention.

DESCRIPTION OF NUMERALS

1: gas lead in pipe

2: first quenching chamber

3: second quenching chamber

4: water discharging pipe

5: water discharging pipe out of the tower

6: refrigerator

7: acid controlling valve

8: acid supply pipe

9: water supply pipe

10: water discharging pipe

11: packed bed

12: water discharging pipe out of the tower

13: refrigerator

14: pH controlling meter

15: water supply pipe

16: gas transferring pipe

17: spray nozzle

18: gas transferring port

19: gas lead in port

20: spray nozzle

BEST MODE FOR CARRYING OUT THE INVENTION <EXAMPLE 1>

The multi-story quenching tower disclosed in FIG. 1 was used. Thequenching tower had a diameter of 2.7 m and a height of 9.7 m. In thefirst quenching chamber (2) of the tower, spray nozzles (17) werearranged on the end part of a pipe (9) at the two-dimensional density of2.3 nozzles/m² for the sectional area of the quenching tower. Thedistance from the tip of the nozzle (17) to the gas leading port (19)was 0.5 m. The type of the nozzles was a hollow cone type.

Propylene, ammonia and molecular oxygen were reacted in a reaction towerto obtain a reacted gas with the composition as follows:

    ______________________________________    < ingredient >       < vol. % >    ______________________________________    acrylonitrile        6.5    ammonia              0.5    propylene and propane                         0.4    acetonitrile         0.3    hydrocyanic acid     1.2    non-condensed gas    63.5    water vapor          27.5    other materials (acrolein,                         0.1    high boiling point compounds, etc.)    ______________________________________

The gas was introduced through the gas lead in pipe (1) into the firstquenching chamber (2) at 20 T/Hr. The linear velocity of the reacted gaswas 0.66 m/sec. The mass plow rate of ammonia contained in this reactedgas was 55 kg/Hr.

Water at a temperature of 85° C., to which sulfuric acid was added so asto adjust the pH value of the water to 5.5, was sprayed from the spraynozzles (17) in an amount of 7.4 T per 1 T of the reacted gas at 148T/Hr. The water was sprayed all over the section of the first quenchingchamber (2).

93% of the ammonia contained in the reacted gas was neutralized andcollected in the first quenching chamber (2). The water was dischargedout of the tower at 0.8 T/Hr.

The reacted gas treated in the first quenching chamber (2) wastransferred to the second quenching chamber (3) through the gastransferring port (18). The packed bed (11) of the second quenchingchamber (3) was filled with porcelain Raschig rings and the spraynozzles (20) were equipped in the same manner as in the first chamber(2). Water at temperature of 37° C., whose pH value was adjusted to 5.5with sulfuric acid, was supplied at 180 T/Hr from the spray nozzles(20).

The 7% of the ammonia remaining in the reacted gas was completelycollected in the second chamber (3); therefore, the gas exhaustedthrough the transferring pipe (16) did not contain ammonia. When thesystem in the tower reached its equilibrium state, the pressure at thebottom of the tower was 0.5 kg/cm² G.

<Comparative Example 1>

The reacted gas with the composition shown in Example 1 was purified inthe same manner as in Example 1 except that the two-dimensional densityof the nozzles was 3.7 nozzles/m² and the amount of water supplied tothe first chamber was 3.9 T per 1 T of the reacted gas. Only 77% of theammonia was removed from the reacted gas, ie, the ammonia level intransferring pipe (16) was 23% of the ammonia level in lead-in pipe (1).

<Comparative Example 2>

The reacted gas with the composition shown in Example 1 was purified inthe same manner as in Example 1 except that the two-dimensional densityof the nozzles was 1.9 nozzles/m². Only 88% of the ammonia was removedfrom the reacted gas, ie, the ammonia level in transferring pipe (16)was 12% of the ammonia level in lead-in pipe (1).

INDUSTRIAL APPLICATION

According to the method of the present invention, collectability ofammonia is improved so that the yield of acrylonitrile can be increasedand trouble, such as clogging in the tower due to the high boiling pointcompounds, is avoided.

What is claimed is:
 1. A method for preparing acrylonitrile in thepresence of a catalyst comprising: reacting ammonia, molecular oxygenand at least one reactant chosen from the group consisting of propyleneand propane to form a reacted gas; and purifying the reacted gas in amulti-story quenching tower by supplying the reacted gas into a firstquenching chamber of the quenching tower, wherein said first quenchingchamber comprises a water supply pipe equipped with nozzles at atwo-dimensional density of at least 2 nozzles/m² of the cross-sectionalarea of the quenching tower, and contacting the reacted gas with atleast 5000 Kg of water fed from the nozzles per 1000 Kg of the reactedgas.
 2. The method for preparing acrylonitrile according to claim 1,wherein the reacted gas is supplied at a linear velocity of from 0.1m/sec to 0.9 m/sec into said first quenching chamber.
 3. The method forpreparing acrylonitrile according to claim 1, wherein at least one spraynozzle is selected from the group consisting of a full cone type nozzleand a hollow cone type nozzle.
 4. The method for preparing acrylonitrileaccording to claim 1, wherein the density of nozzles is from 2 to 8nozzles/m².
 5. The method for preparing acrylonitrile according to claim4, wherein the density of nozzles is from 2 to 5 nozzles/m².
 6. Themethod for preparing acrylonitrile according to claim 1, wherein from5000 Kg to 20,000 Kg of water is fed from the nozzles per 1000 Kg of thereacted gas.
 7. The method for preparing acrylonitrile according toclaim 1, wherein the reacted gas is supplied to the first quenchingchamber by a gas lead-in port (19) and wherein the distance between thespray nozzles (17) and the gas lead-in port (19) is 0.5 meters or more.8. The method for preparing acrylonitrile according to claim 7, whereinthe distance between the spray nozzles (17) and the gas lead-in port(19) is 1.0 meters or more.
 9. The method for preparing acrylonitrileaccording to claim 1, wherein the reacted gas is supplied to the firstquenching chamber at a linear velocity of from 0.10 to 0.90 m/sec. 10.The method for preparing acrylonitrile according to claim 9, wherein thereacted gas is supplied to the first quenching chamber at a linearvelocity of from 0.50 to 0.80 m/sec.